GENE INTERACTIONS.

Here, we study how genes may affect or relate with one another. There exist a wide range of gene interaction but we will be interested in just a few. They include;

EPISTASIS.

This is a situation where the presence of a particular gene masks the presence of another gene in the phenotype. It is sometimes said that the expression of a gene, A, is dependent on another gene, B. There exist a variety of epistasis genes in nature. If a dominant allele from gene A suppresses the expression of gene B, it is referred to as dominant epistasis. If the suppression is done by a recessive allele, it is referred to as recessive epistasis. There exist at least two others but we’ll dwell on these for now. Let’s use the mice as an example. In most mammals, fur colour is determined by two (2) genes. In mice recessive epistasis in observed in their fur colour. There is a gene A that codes for colour: Agouti when the dominant allele is present and black in a double heterozygous state. Nonetheless, the gene A depends on another gene B which determines if there should be colour or not. In the presence of the dominant allele, the gene A is considered and if the gene B is double recessive, the mouse is albino.

The picture below shows the comparison between a normal Mendelian expectation and an epistatic expectation. Notice the difference the number of phenotypes expected.

You can also read on: Mendelian genetics

POLYGENES.

Polygenes are often mistaken with multiple alleles. As seen in the previous part, a multiple allele system is observed when there’s more than one two types of alleles that work independently for a particular trait and are found on the same locus. In the case of polygenes, two or more genes come together to produce a unique result. This is one of the reasons for continuous variation. An example of the multiple allele system is the ABO blood group in humans. In the case of polygenes, we have skin colour and height and many others. As a matter of fact, polygenes are much more common, especially in more advanced species than multiple alleles.

EVOLUTION.

It is important to remember that studies on genetics did not begin with Mendel and neither did it end with him – reason for the discovery of non-Mendelian genetics. Long before Mendel’s research, evolutionist tried to explain variation. The most prominent and most accepted till date is that of Charles Darwin in his book titled “On the Origin of Species”. Though the first five (5) editions of this book did not contain the word “evolution”, it important to know that it was the topic of his writings. It is equally worthwhile knowing that this evolution will not even be discussed if not for variation.

Before Mendel, Charles Darwin proposed the theory of NATURAL SELECTION which explains that some traits are favoured by natural phenomena. The organisms that nature selects will survive and breed, thereby producing organisms that are very similar to their kind. These evolutionist were satisfied with the fact that offspring will naturally find their characteristics somewhere between their parents. (Mendel did not prove otherwise). He used Giraffes to explain. Darwin said, during reproduction there would be a possibility for some giraffes to be born with long necks. These species would have an equal chance like any other for survival except if influenced by nature (they can better access food for example) as these species mate, the probability of producing long-necked offspring becomes greater and hence the population of giraffes with long necks will start increasing in the population. On the contrary, short-necked giraffes will be reducing in the population.

The Hardy–Weinberg Principle.

After Mendel, some Biologist got even more interested in studying evolution. As a result, two people in 1908 independently solved the puzzle of why genetic variation persists - G. H. Hardy, an English mathematician, and G. Weinberg, a German physician. They pointed out that the original proportions of the genotypes in a population will remain constant from generation to generation, as long as the following assumptions are met:

• The population size is very large.
• Random mating is occurring.
• No mutation takes place.
• No genes are input from other sources (no immigration takes place).
• No selection occurs.

Such principles are never met in a natural population but the principles form the basis for the calculations of gene frequencies in a population.

Dominant alleles do not, in fact, replace recessive ones. Because their proportions do not change, the genotypes are said to be in Hardy–Weinberg equilibrium.

The Hardy-Weinberg Principle states that: in a large population mating at random and in the absence of other forces that would change the proportions of the different alleles at a given locus, the process of sexual reproduction (meiosis and fertilization) alone will not change these proportions.

In algebraic terms, the Hardy–Weinberg principle is written as an equation. Consider a population of 100 cats, with 84 black and 16 white cats. In statistics, frequency is defined as the proportion of individuals falling within a certain category in relation to the total number of individuals under consideration. In this case, the respective frequencies would be 0.84 (or 84%) and 0.16 (or 16%). Based on these phenotypic frequencies, can we deduce the underlying frequency of genotypes? If we assume that the white cats (16) are homozygous recessive for an allele we designate b, then the black cats carry the dominant allele and are therefore either homozygous dominant BB or heterozygous Bb. To this effect we can calculate the allele frequencies of the two alleles in the population from the proportion of black and white individuals. To do this, we let the letter p designate the frequency of one allele and the letter q the frequency of the alternative allele. A detailed analysis is shown in the table below.

Consider a gene with alleles “Aa” such that “A” is dominant over “a”.

Let p be the frequency of allele A.

Let q be the frequency of allele a.

NOTE:

• Frequency is the occurrence of anything with respect to a sample space and hence, can never be greater than 1 or 100%.
• There exist only two alleles
• For a particular trait, with only two alleles, three genotypes are possible.

Observe the table below for a given population with the gene, ‘Aa’.

Genotype	1st Allele	2nd Allele	Frequency
AA	A	A	pp=p2
Aa	A	A	pq=pq
aA	A	A	qp=pq
aa	A	A	qq=q2

The frequency of genotypes above represents the total population: p2+2pq+q2=1

Remembering that there exist only 2 alleles per gene: p+q=1

---

Example 5:

---

Let’s evaluate the example discussed above concerning the black and white cats. Let’s define the problem.

Calculate;

1. The frequency of the recessive allele.
2. The frequency of the dominant allele.
3. The frequency of individuals homozygous for the gene.
4. The frequency of the heterozygotes.

Solutions.

Let the allele ‘A’ be p

Let the allele ‘a’ be q

Genotype	Number	Frequency (decimal)	Frequency (percentage)
AA + 2Aa + aa	100	1	100
AA + 2Aa	84	0.84	84%
aa	16	0.16	16%

(We will work in decimals)

Hence, pp + 2pq + qq = 1…………. (1)

           pp + 2pq = 0.84…………….. (2)

          qq = 0.16 ……………………. (3)

From (3): q = root (0.16)

                q = 0.4

But    p + q = 1 ……………………. (4)

          p + 0.4 = 1

         p = 1 – 0.4

       p = 0.6

Therefore;

1. The frequency of the recessive allele is f(a) =  0.4.
2. The frequency of the dominant allele is f(A) =  0.6.
3. The frequency of individuals homozygous for the gene is f(AA + aa) = 0.6*0.6 + 0.4*0.4 = 0.36 + 0.16 = 0.52.
4. The frequency of the heterozygotes is f(2Aa) = 2*0.6*0.4 = 0.48.

---

Exercise 1

---

In a given population, some people are unable to taste phenylthiocarbamite while others experience a bitter taste when placed on the tip of their tongue. The allele for taste is dominant and represented, T and the allele for non-tasters is recessive and represented, t. The following results were obtained from the inability to taste phenylthiocarbamite.

Phenotype	Percentage	Frequency
Tasters	71.2%	0.712
Non-tasters	28.8%	0.288

Calculate;

1. The frequency of the recessive allele.
2. The frequency of the dominant allele.
3. The frequency of individuals homozygous for the gene.
4. The frequency of the heterozygotes.

---

---

More exercises will be uploaded in Part 7.

Understanding the deviations to Mendel's laws of inheritance helps to answer and explain the most pertinent questions of variation and evolution which will be discussed in the next part of our studies, Part 5.

You can equally find some in past Cameroon GCE Advanced and Ordinary level Biology Papers.

Find Cameroon GCE Ordinary level papers here.

Find Cameroon GCE Advanced level papers here.

---

---

Links to previous parts:

A level Genetics Part 1 of 7.

A level Genetics Part 2 of 7.

A level Genetics Part 3 of 7.

A level Genetics Part 4 of 7.

---

---

Note written by BUO GENESIS KELLY

Last edited on the 27/07/2023

---

---

You can interact with us and other mates on our Facebook group via the following link.

Alstug Facebook Group.

---

---